An electric double layer capacitor has a high withstand voltage, a large capacitance, and a high reliability with respect to rapid charge and discharge. Therefore, it is used in many fields. For positive and negative electrodes of a general electric double layer capacitor, a polarizable electrode mainly including activated carbon is used. Furthermore, the withstand voltage of an electric double layer capacitor is 1.2 V when an aqueous electrolyte solution is used and 2.5-3.3 V when an organic electrolyte solution is used. Since the energy of an electric double layer capacitor is in proportion to the square of the withstand voltage, energy is higher when an organic electrolyte solution having a high withstand voltage is used than when an aqueous electrolyte solution is used. However, the energy density of an electric double layer capacitor using an organic electrolyte solution is 1/10 or less of that of a secondary battery such as a lead storage battery. Therefore, further improvement of the energy density is necessary.
In such circumstances, an electric double layer capacitor has been proposed, in which an electrode using a carbon material capable of absorbing and releasing lithium ions is used as a negative electrode and this carbon material is allowed to absorb a lithium ion in advance. Such an electric double layer capacitor is disclosed in, for example, Patent Document 1. Note here that this capacitor uses a polarizable electrode as a positive electrode, and the polarizable electrode mainly includes activated carbon. As a method for allowing a negative electrode to absorb a lithium ion, the following three methods are disclosed.
1) A carbon material and lithium powder are mixed with each other to produce a negative electrode. This negative electrode is immersed in an electrolyte solution, so that lithium is ionized and lithium ions are chemically absorbed by the carbon material.
2) A negative electrode produced by using a carbon material is immersed in an electrolyte solution in a state in which the negative electrode is brought into contact with a lithium foil, so that the lithium foil is ionized and lithium ions are chemically absorbed by the carbon material.
3) A negative electrode produced by using a carbon material and an electrode including lithium are immersed in an electrolyte solution, and an electric current is allowed to flow between the negative electrode and the electrode, so that a lithium ion is electrochemically absorbed by the carbon material.
The above-mentioned electric double layer capacitor has an advantage of having a high withstand voltage and a large capacitance, and is capable of carrying out rapid charge and discharge. However, an operation for allowing a carbon material to absorb lithium ions by a chemical or electrochemical method in advance is complicated, and much man-hour or cost is required. The carbon material is capable of absorbing and releasing lithium ions. Furthermore, it is difficult to obtain an excellent performance stably.
On the other hand, other than an electric double layer capacitor, a lithium ion secondary battery has been developed as a power source capable of charging and discharging a large electric current. A lithium ion battery has a higher voltage and higher capacity as compared with an electric double layer capacitor. However, a lithium ion battery has a higher resistance and remarkably short lifetime by a charge and discharge cycle in a large current as compared with an electric double layer capacitor.    Patent Document 1: Japanese Patent Unexamined Publication No. H9-55342